The SIR provided D,L- [Ring- "N2]Histidine; Ig Difference infrared spectroscopy can be used to identify structural changes that occur in electron transfer proteins upon charge separation. In photosystern II, the photosynthetic oxygen evolving complex, a redox active tyrosine residue, Z, plays an important role in the electron transfer events that precede oxygen evolution. A stable tyrosine radical is also present in the enzyme; this second redox active tyrosine, D, has no known function. We are using vibrational spectroscopy to investigate the structural differences between D and Z. We have obtained the vibrational difference spectrum associated with the oxidation of each tyrosine. This can be done because the decay kinetics of the two radicals differ by many orders of magnitude. The spectra associated with the oxidation of D and Z are different, and the observed spectral differences are consistent with the conclusion that there is a difference in hydrogen bonding to the phenol oxygens of the two tyrosines. Such a difference in hydrogen bonding could help to explain the observed functional differences between D and Z. Use of 180 tyrosine is an important component of our studies, since it will allow us to label the tyrosine residues in vivo. In turn, this labeling will allow us to definitively assign lines in the vibrational spectrum to the C-0 stretch of the radical and to the C-OH stretch of the neutral residue. These studies are in progress.